A major portion of all proteins synthesized in eukaryotic cells are synthesized on the cytosolic surface of the endoplasmic reticulum (ER). Before these immature proteins are distributed to other organelles in the cell or are secreted, they must be transported into the interior lumen of the ER where post-translational modifications are performed. These modifications include formation of disulfide bonds, and N-linked glycosylations.
The mechanism for this translocation process involves the recognition of an N-terminal signal peptide on the elongating protein. The signal peptide directs the protein and attached ribosome to a receptor on the ER membrane. The polypeptide chain passes through a pore in the ER membrane into the lumen while the N-terminal signal peptide remains attached at the membrane surface. The process is completed when the signal peptidase, located inside the ER cleaves ,the signal peptide from the protein and releases the protein into the lumen.
Signal peptidases are found in all prokaryotic and eukaryotic cell types and have been well characterized from yeast and dog. They exist as a multi-subunit signal peptidase complex (SPC) in both species. The yeast SPC consists of four polypeptides of 13, 18, 20, and 25 kDa in size, while the canine protein contains five polypeptides referred to by size as SPC12, SPC18, SPC21, SPC22/23, and SPC25 (Kalies, K-U and Hartmann, E. (1996) J. Biol. Chem. 271(7):3925-29; Shelness, G. S. and Blobel, G. (1990) J. Biol. Chem. 265:(16):9512-19). All five of the canine subunits contain hydrophobic regions that span the ER membrane. SPC18, SPC21, and SPC22/23 present a short N-terminal segment of their polypeptide chain to the cytoplasmic side of the ER membrane, while the larger, C-terminal portion of the molecule is contained on the lumen side. SPC12 and SPC25 appear to be U-shaped, with the middle of the molecule anchored in the membrane and both the N- and C-terminal regions extended on the cytoplasmic side of the membrane (Kalies and Hartmann, supra). SPC12 and SPC25 may function in fixing the SPC to its proper position on the ER membrane, or in recruiting the SPC to the translocation site when the translating ribosome becomes membrane bound. SPC18 and SPC21 are believed to contain catalytic activity and function in signal peptide cleavage. They represent a novel protease family having serine as a key amino acid in the active site (Kalies and Hartmann, supra; van Dijl, J. M. et al. (1995) J. Biol. Chem. 270(8):3611-18).
The discovery of polynucleotides encoding a novel microsomal signal peptidase subunit and the molecules themselves satisfies a need in the art by providing new diagnostic or therapeutic compositions useful in treating or preventing cancer, smooth muscle disorders and immunological disorders.